The invention relates to anti-skid systems for motor vehicles. Anti-skid systems or sometimes called anti-wheel-lock systems usually include a transducer associated with at least one vehicle wheel which generates signals related to the regularity of rotation of the wheel and feeds them to a processor that initiates appropriate pressure decreases at the wheel which tends to lock up. Normally, the propagation of signals to the pressure actuators is subject to an inherent time filtering, i.e. the system does not respond to rapidly repeated actuation signals due to its inherent filtering characteristics. This inertia toward the propagation of actuation signals, i.e. filtering, is actually necessary so as to prevent undesired brake pressure decay due to erroneous or spurious signals or erratic vehicle motions. The pressure decrease signal is tested for a given duration before it is actually used to lower the braking pressure. A kind of filtering against spurious signals is also provided by the fact that genuine signals do not occur until relatively high thresholds of actuation have been passed, for example wheel acceleration or deceleration and wheel slippage.
The filtering time constant and the high actuation thresholds also affect the period of the control loop cycle. This inherent control period or frequency also depends on the condition of the road and is thus a variable. However the inherent period of the control process may not be made arbitrarily small, i.e. the inherent control frequency must not exceed a certain limit which is generally set by the resonant frequency of the vehicle chassis.